Understanding the molecular mechanism by which the signal of the presence of an antibiotic is transduced from outside to inside the bacterial cell is of fundamental interest for the β-lactam antibiotic resistance problem, but remains difficult to accomplish. No approach has ever addressed entire penicillin receptors in a membrane environment. Here we describe a method to investigate the purified Bacillus licheniformis BlaR1 receptor –a membrane-bound penicillin receptor involved in β-lactam resistance– embedded into a lipid bilayer in absence or presence of penicillin. By selecting a mutated receptor blocked in its signal transduction pathway just after its activation by penicillin, we revealed the very first step of receptor signalling by unfolding the receptor from its C-terminal end by AFM-based single-molecule force spectroscopy. We showed that the presence of the antibiotic entails significant conformational changes within the receptor. Our approach opens an avenue to study signal-transduction pathways mediated by membrane-bound proteins in a membrane environment.